Field of the invention
The present invention relates to a metal or metal oxide porous thin film having a three-dimensional open-network structure, which is used for a gas sensor, a biosensor, a battery-capacitor, a fuel cell, a solar cell, a chemical catalyst, an antibacterial filter, etc., and, more particularly, to a metal or metal oxide porous thin film having a three-dimensional open-network structure, which is manufactured through a dry process instead of a wet process and has mesopores, and a method for manufacturing the same.
Related Art
A porous thin film is used for a gas sensor, a biosensor, a solar cell, a secondary battery, a fuel cell, an environmental filter, a catalyst, etc. The porous thin film used at present is manufactured by a wet process such as a sol-gel method, a template method, a coating method.
In a gas detection sensor, when a gas contacts the surface of an oxide semiconductor, which is the main constituting element of a gas detection electrode, a change occurs in the thickness of the depletion layer of the surface and the change in the electric conductivity occurring therefrom is measured and the concentration of the gas is detected. The gas sensor market (including the biosensor market) is currently rapidly expanding from 13 billion US dollars in 2012 to 26 billion US dollars in 2019 (BCC Research (2013.04, www.bccresearch.com)) and the domestic gas market in Korea is expected to continuously grow on a steady pace from 60 billion Korean Won in 2009 to 140 billion Korean Won in 2019 by an annual growth rate of 7.2%.
A sol-gel method is a method for forming pores by an alkyl group, which is a metal ligand, and evaporation of a solvent. The method can only form oxides and has disadvantages in that it is difficult to control pore distribution and a long-term drying is required for obtaining a uniform membrane without a crack. Additionally, the method requires heat treatment to improve crystallinity, during which the porosity is decreased while increasing particle size.
A template method is a method where a template is inserted during the manufacture of a thin film and pores are formed by removing the template, and in general, pores are formed by a sol-gel method or coating method. The template method has an advantage in that it enables the realization of a regular pore structure but the method has a problem in that it requires high-temperature heat treatment or acid treatment to remove the template. Additionally, the method has a disadvantage in that the processing cost is increased because it is impossible to recover the template.
A particle coating method is a method for forming pores using the space between the original pores of particles and the particles themselves. The method has an advantage in that it is possible to control pore distribution according to the porosity and size of pores, but the method also has problems in that it requires the use of a binder and a solvent, requires heat treatment for their removal after the manufacture of the film, and also porosity is decreased because the particles grow and bind during the heat treatment. In the conventional method of manufacturing the porous thin film, a template method is a method where a template is inserted during the manufacture of a thin film and pores are formed by removing the template, and in general, pores are formed by a sol-gel method or coating method. The template method has an advantage in that it enables the realization of a regular pore structure but the method has a problem in that it requires high-temperature heat treatment or acid treatment to remove the template. Additionally, the method has a disadvantage in that the processing cost is increased because it is impossible to recover the template.
That is, the wet processes as described above essentially require a heat treatment process and subsequently there is an inevitable problem of loss of pores due to heat treatment. As a technology to manufacture a porous thin film by a dry process to replace the wet process, Korean Patent Application Publication No. 10-2013-0023011 discloses a method for fabricating a porous thin film which includes forming a thin film from two or more elements followed by selectively removing particular elements using a dry lithography, and a porous thin film structure manufactured therefrom. According to the above technology, the entire process of manufacturing the porous thin film structure consists of only a dry process and it thus has advantages in that the method is simpler in terms of procedural management compared to a case when a wet process such as the conventional electrodeposition method or selective dissolution method, etc., has a less burden with regard to the environment, and a large-scale production is possible. Additionally, the above technology not only enables an easy control of porosity but also maintains a constant porosity and thus a mesoporous thin film structure that exhibits a reproducible sensitivity can be manufactured when it is used as a sensor. However, the procedure had problems in that it is complex because the procedure includes a process of selectively removing elements thus increasing the manufacturing cost, the pore uniformity does not meet the required standard, and the range of controlling porosity is narrow.